Historically, creped tissue has been produced by adhesion against Yankee dryers or other heated drums to first dry the tissue, followed by creping with a doctor blade. More recently, the use of paired steel bands having a temperature differential between them has been proposed as a means of drying tissue, after which the tissue can be removed from one of the bands by creping or other means to cause the tissue to become foreshortened. An example of such a proposal is found in the PCT publication WO 99/32716, xe2x80x9cProcess and Apparatus for Making Foreshortened Cellulosic Structure,xe2x80x9d by C. A. McLaughlin et al., published Jun. 1, 1999, the U.S. counterpart of which, Ser. No. 08/994,927, filed Dec. 19, 1997, is herein incorporated by reference. The McLaughlin et al. reference discloses methods for foreshortening tissue which includes creping from tissue dried on a steel band in what is a version of a commercial drying concept known as the CONDEBELT(trademark) marketed by Valmet Corp. (Finland). Related technology, in which differential temperature was used to remove moisture by a combination of and are described in a variety of patents and publications, including U.S. Pat. No. 4,112,586 issued on Sep. 12, 1978; U.S. Pat. Nos. 4,506,456 and 4,506,457 both issued on Mar. 26, 1985; U.S. Pat. No. 4,899,461 issued on Feb. 13, 1990; U.S. Pat. No. 4,932,139 issued on Jun. 12, 1990; and U.S. Pat. No. 5,594,997 issued on Jan. 21, 1997, all foregoing patents issued to Lehtinen; U.S. Pat. No. 4,622,758 issued on Nov. 18, 1986 to Lehtinen et al.; and, U.S. Pat. No. 4,958,444 issued on Sep. 25, 1990 to Rautakorpi et al., all of which are herein incorporated by reference.
A potential limitation in the embodiments discussed above is the problem of nonuniformity in temperature, web adhesion, and topography (flatness) of the steel bands, particularly in the cross direction, whereby web breaks may occur during creping or other foreshortening operations, or whereby nonuniform product may be obtained. While the CONDEBELT(trademark) system may provide improved temperature uniformity across the majority of the web in current machines, the potential for temperature variability may increase with increases in machine width or speed, resulting in more severe cross-direction temperature gradients. For example, cooler edges and hotter central portions of the belt may cause deflection of one segment of the steel band relative to other segments of the steel band, thus presenting a non-uniformity or distortion of the flat surface of the steel band from which the tissue would be creped. Uniformity of the surface of the steel band beneath the tissue is important when using a straight creping blade. Improved mechanisms for maintaining good thermal uniformity in the cross-direction and/or geometric uniformity of the surface of the steel band (flatness, for example) are needed, not only for the steel bands, but the support structures for the steel bands and the structures that support pressure chambers and other components of the system.
Further, the surfaces of the steel bands at high temperature, when pressed against a low-basis weight web such as tissue, are likely to present challenges in web removal. A sudden drop in applied pressure as the web leaves the compression zone of a CONDEBELT(trademark) press may result in delamination, when the internal steam pressure in the web is suddenly no longer balanced by externally applied pressure. Controls are needed to prevent delamination. A further problem associated with removal of the web from the CONDEBELT(trademark) system is that some parts of the web may adhere strongly to the surface of the steel band while other parts of the web are adhered less strongly at the point of removal from the surface of the steel band, resulting in the potential for poor creping if a crepe blade is used to remove the web, or resulting in web breakage if the web is pulled off the surface of the steel band, as could occur if differential velocity transfer to a slower moving web were used. The adhesion to the surface of the steel band is not only a function of temperature and the uniformity of any adhesives applied and of any pressing force used to contact the web to the surface of the steel band, but is also affected by the surface energy of the surface of the steel band, which in turn can be strongly dependent on oxidation of the surface of the steel band or the build up of mineral deposits or the build up of other chemicals on the surface of the steel band. Improved surface treatments of the steel bands are needed to promote uniformity of the surface of the steel band, to prevent oxidation or other sources of nonuniformity in surface energy, and to promote good web release, especially when creping is not used to remove the web.
Further still, nonuniform sheet properties and runnability problems may occur unless measures are taken to provide uniform, intimate contact of the web to the steel band. When a moist web contacts a heated metal surface, the potential for blistering (steam pockets causing portions of the web to move away from the heated surface or to have nonuniform contact against the surface) or other uniformity problems needs to be considered and prevented. Thus, there is a need for process improvements over what has been proposed to ensure that good, intimate contact between the tissue and the contacting steel band is achieved, preferably prior to entering the dryer section or immediately thereupon.
Also neglected in the art is the utility of metal band systems for the production of latex-reinforced tissue, particularly for double-creped products.
While it is known that foreshortened cellulosic webs can be produced using dryers having moving belts or other moving press surfaces that are substantially parallel for a distance, wherein the dryer applies a temperature differential across the thickness of the web, such devices can be improved with means for reducing heterogeneity in the web. Undesired heterogeneity can be due to poor cross-directional control of drying, web properties, belt tension, belt or web topography (especially when creping is used), and the like, or can be due to nonuniform or poorly controlled surface or thermal properties of the belts or other moving press surfaces. Reduced heterogeneity can be achieved with a control system for detecting and reducing cross-directional variability. The control system can respond to sensors measuring temperature or flatness of the web or a moving belt, surface topography of the web, local tension in the web, elastic modulus of the web, and the like. Alternatively or in addition, reduced heterogeneity can promoted with durable coatings on at least one of the moving press surfaces (e.g., a metal press belt that contacts the cellulosic web) to improve heat flux into the web, contact of the web to the press belt, release of the web from the press belt, or other factors affecting the drying, foreshortening, or material properties of the web.
Heterogeneity can also be occur when there is incipient or fully developed delamination of a web upon exiting a compression zone with a temperature differential, or in general by a sudden change in applied pressure while heated. Such heterogeneity can be reduced by control of the depressurization of the web or the applied temperature of the web prior to exiting a compression zone, such as by providing a decompression zone for a more gradual change in pressure, or by providing an intermediate open zone before completely exiting the drying device which can permit the release of steam in the web (and also partly cool the web) or can permit for measurement or treatment of the web prior to completion of drying. Such treatment can refer to application of profiled heating or cooling of the web, and/or application of additives. Other strategies involving cross-directional control of properties of the web, the moving press surfaces, the applied pressure or heat flux, and so forth, can also be useful in reducing delamination or other problems associated with intense drying operations.
Improved press belt structures can also lead to improved drying performance or more uniform web properties.